Mysterious Ditches along the Mulpus - prehistoric or not?

Look how the river, for one bend, runs parallel to the ditches. Since this is very flat, I think the "Stupid Sheet" might apply [link to the right] and, if this was prehistoric, would suggest a preoccupation with the 260 degree lunar standstill. Of course the ditches could be historic but I am having a hard time imagining a "practical" reason to straighten out a brook.

Glancing at the Fort Devens stone wall map [link to the right] I see plenty of walls running at that angle - parallel with a stretch of the Mulpus. Why is a brook parallel with a stone wall? Come to notice it, what the heck is up with that map coordinate line? 

Its a mystery! They could do a TV show.   

Update: I guess there was a lot of ditching in colonial times and, if I take into account that the Devens map has true north as vertical, then this is a true east-west direction. Hard to tell what the coords are for the satellite view.